Method of making a stacked thin film assembly

ABSTRACT

A stacked film assembly for use as wiring in a semiconductor device having a bottom film (CVD-W film) 33 and a top film (Al alloy film) 12, where the surface roughness (Ra) of the bottom film is less than 100 Å and the crystal orientation of the top film formed on this surface is controlled, a CVD method for the making thereof, and a semiconductor device in which the stacked film assembly is employed. Even when there is no lattice matching of the bottom film and the top film, crystal orientation of the top film can be sufficiently controlled to provide a targeted face ((111) face with aluminum film), and in particular it will be possible to readily form a stacked film assembly having a satisfactory barrier function as well as sufficient EM resistance and with good film formation.

This invention pertains to a stacked film assembly (particularlyaluminum wiring that has a barrier metal layer as the underlying film),its method of formation and a semiconductor device (for example, adynamic random access memory: DRAM) employing same.

BACKGROUND OF THE INVENTION

It is already known that when aluminum thin film is used as wiringmaterial, the direction of the crystal particles and crystal particlediameter affect deterioration, e.g., wire breakage due toelectromigration (abbreviated EM hereafter).

In short, the life of aluminum wiring until it disconnects because of EM(EM resistance): MTF (mean time to failure) is expressed by thefollowing formula, based on experience, of S. Vaidya.

    MTFα(S/σ.sup.2) log (I.sub.111 /I.sub.200).sup.3 . . . (1)

Where, S: aluminum polycrystal grain diameter

σ: variation in this crystal grain diameter

I₁₁₁ : (111) face X-ray diffraction intensity

I₂₀₀ : (200) face X-ray diffraction intensity

According to the formula (1), as the crystal grain diameter increases,variation becomes smaller, and the fact that crystal grain orientation(phenomenon where crystal direction follows a specific direction) iscontrolled by the (111) face crystal direction is important forimproving EM resistance.

Prior to this, aluminum wiring has used a construction, as shown in FIG.30, where it is applied to contact hole 4 of insulation layer 3 onsemiconductor substrate 1 as a single layer film 12 and conducts oninsulation layer 3 by connection to a specific semiconductor region 22.With this type of single layer aluminum wiring 12, however, aluminumatoms diffuse toward the semiconductor substrate during annealing orheating, forming an Al-Si alloy. Spike alloy 10 shown in FIG. 30 isreadily produced, and if this reaches pn junction 11, there will be aproblem with shorting between wiring 12 and substrate 1. Because ofthis, even if the aluminum crystal diameter is controlled as describedabove, EM resistance will be insufficient.

So, as shown in FIG. 31, barrier metal film 13, to prevent aluminum atomdiffusion, is provided beneath aluminum film 12, and the formation ofwiring with this stacked structure having aluminum film 12 and barriermetal film 13 is widely used.

The use of titanium nitride (TiN), for example, as the barrier metalfilm 13 is already known. Along with the barrier function of TiN, Alcrystal orientation is controlled by controlling the TiN crystalorientation. In other words, this takes advantage of certaincharacteristics. By controlling TiN crystal orientation to (111), Alcrystal orientation will follow the (111) face orientation. With TiN andAl made of the same face centered cubic construction: FCC, the latticeconstants of the two are comparatively close, 4.2417 Å for TiN and4.0494 Å for Al, and the Al (111) crystal face will grow in conformityto the TiN (111) crystal face.

This TiN/Al stacked film, has the problems shown in (1)-(6) below,however.

(1) TiN is applied by reactive sputtering or Ti is nitride-treated aftersputtering. In either case, as shown in FIG. 32, when the TiN applied bysputtering or Ti 13 is particularly thin where it adheres to the insideof contact hole 14, defects occur readily, and the material in thisstate cannot function as an underlying layer (barrier metal) for thealuminum film applied as the top layer. If the film is made thicker, itwill adhere together above contact hole 4, as indicated by the imaginarylines, completely blocking contact hole 4. With the top aluminum filmunable to adhere inside contact hole 4, contact defects can easilyoccur. In either case, since sputtering is used, there is a tendency forstep coverage to be poor, and conditions for uniform application arelimited.

(2) Since the work function difference between TiN and n-type siliconand p-type silicon is very large, electrical contact of TiN film 13alone to a silicon substrate (semiconductor region 22 above) isdifficult. To improve this, a metal film, Ti, etc., must be used as anunderlying layer for TiN film 13. Thus, even when TiN is applied only tocontact hole 4, i.e., so-called selective formation (selective-TiN),electrical contact will not be obtained.

(3) In addition, even when TiN is formed from contact hole 4 oninsulation film 3, as a so-called blanket type, electrical contact isnot obtained with TiN alone.

(4) TiN film 13, which serves as the aluminum film underlying layer, hasthe same (111) crystal plane orientation as the aluminum, so crystaldirection may be over-aligned. With annealing or heating, aluminum atomsreadily diffuse toward the semiconductor substrate along the TiN crystalgrain boundaries through the TiN film, and a spike alloy as describedabove may be created.

(5) Controlling TiN crystal orientation to the (111) face requiresspecial conditions. That is, for film formation conditions, TiN isdeposited by controlling substrate bias during reactive sputtering, orTi deposited by sputtering is nitrided by lamp illumination in N2. Inaddition, depending on the degree of nitriding, reproducibility of thecrystal orientation may be insufficient.

(6) The problems as described in the aforementioned (1)-(5) paragraphsoccur not only when TiN is applied to the contact hole, but also whenTiN is applied to the insulation layer with through-holes, whenmultilayer wiring that connects upper and lower wiring layers isapplied. Furthermore, the same unavoidable problems occur even whenwiring is drawn around over the insulation film.

Even in wiring that has a stacked structure other than theaforementioned TiN/Al stacked film, the same problems may occur. Astacked film assembly with a barrier function and sufficient EMresistance with good film formation and that can be formed easily hasbeen desirable.

It is an object of this invention to provide a structure and methodwhere crystal orientation of the top film can be satisfactorilycontrolled, and with which a stacked film assembly with sufficientbarrier function and EM resistance can be formed satisfactorily andeasily, by controlling the properties of the underlying film from aperspective different from that of the prior art.

SUMMARY OF THE INVENTION

In short, this invention pertains to a stacked film assembly made with astacked structure having a bottom film and a top film. The surfaceroughness (Ra) of the bottom film will be less than 100 Å, and thecrystal orientation of the top film as formed on the surface of thebottom film will be controlled.

In investigating stacked film assemblies, particularly using aluminumfilm (actually, these may be alloy films that contain small quantitiesof Si and Cu) as the top film, it has been discovered, in accordancewith the invention, that even when there is no lattice matching of thebottom layer to the aluminum film, as there is with TiN, by keeping thesurface roughness (Ra) of the bottom layer in a specific range, i.e.,less than 100 Å, crystal orientation of the top layer can be controlledto what is targeted ((111) face, with aluminum film).

For example, when tungsten film that can be formed with chemical vapordeposition (CVD-W) is formed as the bottom film, the crystal structureof this CVD-W film will have a body centered cubic construction: BCC(lattice constant of 3.165 Å different from that of the aluminum and itscrystal direction will be a mix of (110) and (200). Regardless of thefact that, with no lattice matching with the aluminum (111) crystalface, control of the aluminum crystal orientation is not possible bycontrolling the crystal orientation of the bottom film as describedabove, it has been established, in accordance with the invention, thatthe aluminum (111) crystal face orientation depends on the surfaceroughness Ra of the CVD-W film that serves as the bottom film.

In short, X-ray diffraction intensity of the top film Al (111) facechanges depending on the surface roughness Ra of the CVD-W film servingas the bottom film, as shown in FIGS. 1 and 2. In particular, when thesurface roughness Ra of the CVD-W film is kept at less than 100 Å (10nm), the X-ray diffraction intensity of the Al (111) face is greatlyincreased. In other words, Al (111) face crystal orientation isincreased, and sufficient EM resistance can be realized.

The fact that the top layer crystal orientation can be controlled tothat targeted by controlling surface roughness Ra, even with a bottomfilm whose crystal faces do not match those of the top film, is based onan epoch making concept that could not have been imagined in the priorart. In particular, if the bottom film is made of CVD-W film, theresults are even more remarkable, as shown in the following (a)-(f).

(a) Since the bottom film is formed by CVD (chemical vapor deposition),satisfactory adhesive ability of the bottom film within contact holesand through-holes will be provided, whether the film is thick or thin.An aluminum atom diffusion prevention function (barrier function) aswell as improved contact ability will be achieved with this bottom film.In particular, it is extremely significant that formation of lowresistance wiring and interlayer connection can be realizedsimultaneously by taking advantage of the excellent characteristics ofaluminum film, which has low absolute resistance, as the top layer, andCVD-W film, which has good step coverage ability, as the bottom layer ina stacked thin film assembly.

(b) For example, WF₆, which can be used when CVD-W film is formed, ismuch more electronegative than silicon. Thus, it is readily adsorbedonto the silicon substrate, and the W-F feed is readily cut off byelectrons fed from the silicon. Due to this, thermal decomposition ofWF₆ is promoted at the silicon surface, W is readily deposited onto thissurface, and CVD-W with good adhesive ability will be deposited on thesemiconductor substrate. Thus, this is particularly effective for theselective formation described above, and an underlying layer is notrequired.

(c) The bottom CVD-W film has no lattice matching to the aluminum, soaluminum atom diffusion along crystal grain boundaries, which is seenwith TiN film as already stated, does not occur. Thus, the spike alloydescribed previously is not created, and the functioning of the bottomCVD-W film as a barrier metal will be sufficient.

(d) CVD-W film can be formed with normal CVD film formation conditions,which can be used to form film easily without applying the specialequipment or treatment typical of TiN film formation. Furthermore,control of the top film orientation can be realized, with goodreproducibility, primarily by only controlling the surface roughness(Ra) of the bottom film.

(e) When the bottom film is formed by CVD, the crystal grain diameter ofthe top film can be kept sufficiently large regardless of thefilm-forming conditions of the bottom layer (particularly changes inbottom film thickness), and this will also be effective for keeping EMresistance satisfactory.

(f) The effects described in paragraphs (a)-(e) can be obtained not onlywith the aforementioned selective formation and blanket film formationapplied to a contact hole, but also for application to through-holesduring multilayer wiring formation. In addition, it is also effectivefor drawing wire around and over the insulation film.

However, when the CVD-W is formed into a blanket-type film, theelectronegativity of WF₆ is close to that of SiO₂, so it is difficult todeposit this onto the SiO₂ with satisfactory adhesion. For this reason,an adhesive film (Ti-W alloy film, for example) should be formed as anunderlying layer for the bottom film.

As already discussed, the stacked film assembly as constructed inaccordance with this invention has outstanding characteristics in thatthe surface roughness (Ra) of the bottom film is specifically less than100 Å; the following means is primarily used to reduce the surfaceroughness Ra in this way to the specified range.

First, the thickness of the bottom film should be reduced, andpreferably should be less than 1000 Å (less than 100 nm). This caneasily be realized by controlling CVD conditions (film formation time,for example). In other words, it is clear, as shown in FIG. 1, that thesurface roughness Ra of the CVD-W film can be kept less than 100 Å inaccordance with this invention if the CVD-W film thickness is less than1000 Å.

Due to the fact that CVD-W film thickness can be reduced in this way,the effect in (g) below can also be obtained, in addition to theaforementioned effects.

(g) Patterning by etching is difficult with a CVD-W/Al stacked filmassembly formed in accordance with this invention, but since the CVD-Wfilm can be made thinner than 1000 Å, etching will be easy, andpatterning precision that is as designed can be achieved.

As other means of controlling the surface roughness Ra of the bottomfilm, the following two methods for film formation conditions with CVDcan be used.

First there is the interconnect method (hereafter called the I.C.method) in which, when a stacked film assembly made of a structure withbottom and top films is formed, a chemical gas (e.g. WF₆) with thestructural elements of the aforementioned bottom layer is fed in, thisis broken down under the circulation gas (hydrogen, for example) feedlimiting conditions, and the bottom layer (e.g. W or tungsten) is formedby chemical vapor deposition of the aforementioned structural elements.

Secondly, there is the nucleation method and the via fill method, i.e.,(NUC.+V.F.) where, when a stacked film assembly made of a structure withbottom and top films is formed, the nucleation method may be firstemployed by feeding in a chemical gas (e.g. WF₆) with the structuralelements of the bottom layer. After this chemical gas is broken downusing a first circulation gas (SiH₄, for example) that is highlyreactive. This nucleation procedure provides for a very low depositionrate of the desired metal as compared with the via fill (i.e. V.F.)method and is employed to nucleate or seed a metal (e.g. W or tungsten)onto a substrate, such as SiO₂, W, TiW, Al--Si--Cu etc. with a very thinthickness such as 500 Å. Following nucleation, the via fill method isemployed where the chemical gas (e.g. WF₆) is broken down under surfacereaction limiting conditions that use a second circulation gas (H₂, forexample) that has low reactivity in place of the first circulation gas.The bottom layer is formed by vapor phase epitaxy of the aforementionedstructural elements.

In other words, as shown in FIG. 2, the surface roughness (Ra) of theCVD-W film formed by the I.C. method or (NUC.+V.F.) method can be keptto less than 100 Å, and satisfactory control of the top layer (111)crystal face can be realized. This corresponds to the trend shown inFIG. 1.

With the I.C. method, the circulation gas should be made rich in H₂relative to the reaction gas, WF₆, and the speed of the reaction can beset by the H₂ feed rate. With the (NUC.+V. F.) method, WF₆ is firstbroken down by the action of the SiH₄ circulation gas in the NUC portionof the method, and a W film is deposited as seed. A second W film willbe deposited on the seed W film by V.F. by breakdown of WF₆ under normalconditions.

In this (NUC.+V.F.) method, since the surface of the W film produced byV.F. is easily roughened, and film formation speed by NUC. can bereduced, the thickness ratio of each W film formed with these methodsshould be controlled. The bottom film should be formed so that the ratioof the thickness of W film produced with NUC. and the thickness of Wfilm produced with V.F. should be (5:5)-(3:7).

Note that, in the stacked film assembly as constructed in accordancewith this invention, it is essential that the surface roughness Ra ofthe bottom film be less than 100 Å, but it is further preferable thatthe surface roughness Ra be less than 65 Å (and correspondingly, thatthe thickness of the bottom film be less than 500 Å). This can beunderstood from the results in FIG. 1 (for films produced by V.F.). Inaddition, the fact that the I.C. method is preferable as the filmformation method for the bottom film can also be understood from theresults in FIG. 2.

The surface roughness Ra of this bottom film will be in accordance withJapanese Industrial Standard JIS 0601 for surface roughness and isdefined in the following way (the same hereafter). In short, Ra (centerline average roughness) is given by the following equation, when themeasured length L is taken from the roughness curve in the direction ofthe center line, the center line is the X axis, the longitudinalmagnification direction is the Y axis, and the roughness curve isexpressed by y=f(X).

    Ra=1/L|f(X)|dx                           (2)

In addition, measurement of the roughness curves was carried outprimarily using a known AFM (atomic force microscope: NANO SCOPE, madeby Digital Instruments Co.).

Crystal orientation and crystal grain diameter of the stacked filmassembly were measured using known X-ray diffraction (XRD) and SEM(scanning electron microscope) techniques.

In addition, this invention also provides a semiconductor device inwhich the stacked film assembly is provided on a semiconductorsubstrate. In particular, the invention can provide a semiconductorintegrated circuit device, e.g., DRAM, having a structure in which astacked film assembly is formed as wiring on an insulation filmcontaining contact holes or through-holes (blanket type), or a structurein which a bottom film of the stacked film assembly is selectivelyformed as the underlying conductor for the top film only in contactholes or through-holes (selective formation type).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing changes in CVD-W film surface roughness andtop film (Al alloy film) (111) X-ray diffraction intensity dependingupon the film thickness of CVD-W film formed by the V.F. method.

FIG. 2 is a graph showing changes in CVD-W film surface roughness andtop film (Al alloy film) (111) X-ray diffraction intensity dependingupon the film thickness of CVD-W film formed by each of various methods.

FIG. 3 is an X-ray diffraction spectrum diagram for CVD-W film thicknessin a stacked film assembly (TiW/CVD-W/AlSiCu) that has a CVD-W filmformed by the V.F. method.

FIG. 4 is an X-ray diffraction spectrum diagram corresponding to CVD-Wsurface roughness in a stacked film assembly (TiW/CVD-W/AlSiCu) that hasa CVD-W film formed by each of various methods.

FIG. 5 is a graph showing changes in the top film (Al alloy film) Alcrystal grain size depending upon CVD-W film thickness in a stacked filmassembly that has a CVD-W film formed by the V.F. method.

FIG. 6 shows SEM photographs showing the top film (Al alloy film) Alcrystal grains when the thickness of a CVD-W film, in a stacked filmassembly that has a CVD-W film formed by the V.F. method, was varied.

FIG. 7 shows SEM photographs which show the surface roughness of CVD-Wfilms formed by each of various methods.

FIG. 8 is an X-ray diffraction diagram produced with the pole graphicmethod and based upon the surface roughness of a CVD-W film in a stackedfilm assembly.

FIG. 9 is an X-ray diffraction spectrum diagram in the transversedirection with the same pole graphics employed with respect to FIG. 8.

FIG. 10 is a descriptive diagram representationally showing the processof growth of Al crystal grains on a CVD-W film.

FIG. 11 is a TEM (transmission electron microscope) photograph of thestacked film assembly.

FIG. 12 is a TEM photograph of the stacked film assembly.

FIG. 13 is a TEM photograph of the stacked film assembly.

FIG. 14 is a TEM photograph of the stacked film assembly.

FIG. 15 is an Al crystal grain size distribution diagram based on TEM ofthe stacked film assembly.

FIG. 16 is an Al crystal grain size distribution diagram based on AFM(atomic force microscope) of the stacked film assembly.

FIG. 17 is a plane image of each type of stacked film assembly usingAFM.

FIG. 18 is a three-dimensional image of a CVD-W film using AFM.

FIG. 19 is a three-dimensional image of a CVD-W film using AFM.

FIG. 20 is a three-dimensional image of a CVD-W film using AFM.

FIG. 21 is a three-dimensional image of a CVD-W film using AFM.

FIG. 22 is a three-dimensional image of a CVD-W film using AFM.

FIG. 23 is a three-dimensional image of a CVD-W film using AFM.

FIG. 24 is an X-ray diffraction spectrum diagram based upon top film (Alalloy film) thickness in each type of stacked film assembly.

FIG. 25 is a graph showing changes in VIA resistance (contactresistance) caused by the thickness of the CVD-W film in the stackedfilm assembly.

FIG. 26 is a cross sectional view of a semiconductor device having astacked film assembly formed as wiring (blanket type).

FIG. 27 is a cross sectional view of a DRAM memory cell with the wiringserving as a bit line.

FIG. 28 is a schematic cross sectional view of a semiconductor devicewith the same wiring formed thereon.

FIG. 29 is a schematic cross sectional view of a semiconductor devicewith the stacked film assembly formed as wiring (CVD-W is theselectively formed type).

FIG. 30 is a schematic cross sectional view of a semiconductor devicehaving a conventional single layer wiring formed thereon.

FIG. 31 is a schematic cross sectional view of a semiconductor devicewith a conventional stacked film assembly formed as wiring.

FIG. 32 is a cross sectional view similar to FIG. 31 and showing a stepcoverage state in the same wiring.

Reference numerals as shown in the drawings:

1 . . . semiconductor substrate

3 . . . insulating layer (SiO₂)

4 . . . contact hole

12 . . . Al alloy film

13 . . . TiN film

22 . . . semiconductor region

31 . . . TiW film

33 . . . CVD-W film

34 . . . reflection prevention film

DESCRIPTION OF PREFERRED EMBODIMENTS

The stacked film assembly based on this invention uses as its basis, asshown in FIG. 26, a stacked structure of CVD-W film 33, formed with thechemical deposition described below, and aluminum film 12, formed withknown sputtering, CVD, or vapor deposition. In addition, it has adhesionor sticking film 31, made of TiW, etc. by known sputtering as theunderlying layer for CVD-W film 33, and is formed over insulation layer3 from contact hole 4.

Based on this invention, surface (boundary with aluminum film 12)roughness Ra of CVD-W film 33 is controlled to less than 100 Å, and itsthickness is less than 1000 Å. Aluminum film 12 is formed with athickness of, for example 5000 Å-6000 Å, and is made of aluminum alloywith a small quantity of Si and Cu mixed in, to prevent alloy spikes andincrease EM resistance. Adhesive film 31 is formed with a thickness of,for example, 600 Å, and may be made from a variety of materials,including Ti-W (specific resistance of 50-60 mΩ-cm), TiN (specificresistance 70-200 mΩ-cm), W (specific resistance approx. 13 mΩ-cm).

    __________________________________________________________________________                  FILM FORMATION                                                                           FILM FORMATION                                                                          EXAMPLE OF CVD-W FILM                      TYPE          CONDITIONS TIME      SPECIFIC RESISTANCE                        __________________________________________________________________________    I.C. METHOD   Pressure = 80 Torr                                                                       1000 Å/17 sec                                    cm                                 8 μΩ                              (FEED DETERMINATION)                                                                        Temperature = 442° C.                                                  Gas Flow Rate =                                                               Ar: 2200 sccm                                                                 N.sub.2 : 20 sccm                                                             WF.sub.6 : 36 sccm                                                            H.sub.2 : 1800 sccm                                             NUC. METHOD   Pressure = 1 Torr                                                                         500 Å/40 sec                                    cm                                 11-12 μΩ                          (SiH.sub.4 REDUCTION)                                                                       Temperature = 442° C.                                                  Gas Flow Rate =                                                               Ar: 300 sccm                                                                  N.sub.2 : 30 sccm                                                             SiH.sub.4 : 5 sccm                                                            WF.sub.6 : 500 sccm                                             V.F. METHOD   Pressure = 80 Torr                                                                        500 Å/11 sec                                    cm                                  9-10 μΩ                          (SURFACE REACTION RATE                                                                      Temperature = 442° C.                                                             1000 Å/17 sec                                    DETERMINATION)                                                                              Gas Flow Rate =                                                                          2000 Å/27 sec                                                  Ar: 2200 sccm                                                                 N.sub.2 : 300 sccm                                                            WF.sub.6 : 60 sccm                                                            H.sub.2 : 500 sccm                                              __________________________________________________________________________

The methods for forming CVD-W film 33 are explained. In the methods,there are the I.C. method, the NUC method, the V.F. method, and a method(NUC.+V.F.) combining the NUC. method and the V.F. method, etc.

In addition, film formation conditions for TiW film 31 and aluminum(AlSiCu) film 12 are as follows.

TiW film 31: pressure=10 mtorr, temperature=200° C., output=2 kW,time=23 sec.

AlSiCu film 12: preheating=200° C. pressure=7 mtorr, output=12 kW,time=25 sec, with no heating during sputtering.

So, CVD-W film 33 was formed of various thicknesses with each of thefilm formation methods, AlSiCu alloy film 12 was layered on the CVD-Wfilm 33, and all measurements were performed.

For the CVD-W/AlSiCu stacked thin film assembly, CVD-W film thicknesswas varied from 2 kÅ to 0.5 kÅ. After AlSiCu sputtered film formation,heat treatment at 450° C. (H₂ atmosphere) for 30 min was applied. The Alcrystal direction in this sample was examined using XRD (Θ-2Θ method),with the results being shown in FIG. 3. Al (111) diffraction intensityshowed a tendency to increase as the CVD-W film became thinner. (110)and (200) diffraction peaks were observed from the CVD-W film, and the(200) diffraction intensity showed a tendency to become smaller alongwith decreased film thickness.

Al crystal grain diameter was measured for the same sample using SEM;the results are shown in FIG. 5, and the SEM observation photographs areshown in FIG. 6. Al crystal grain diameter does not greatly depend onCVD-W film thickness, and was somewhat greater in the 0.5 kÅ sample thanthe others.

Center line average roughness (Ra), measured using SEM photographs andAFM for samples where CVD-W film thickness was varied from 2 kÅ to 0.5kÅ (V.F. method films) and CVD-W film thickness was fixed (1 kÅ) andsurface roughness changed by changing film formation conditions, isshown in FIG. 7. Surface roughness of the CVD-W film (V.F. method film)had a tendency to decrease as film thickness decreased. In addition, itcan be seen that even in the same film thickness, surface roughness canbe controlled by changing film type.

From the results in FIGS. 3 and 7, the relationship between surfaceroughness of the CVD-W film (V.F. method film) where film thickness wasvaried from 2 kÅ to 0.5 kÅ, and Al (111) diffraction intensity of theAlSiCu film formed on these films and heat treated, is shown in FIG. 1.CVD-W film surface roughness decreases with a decrease in CVD-W filmthickness, and Al (111) diffraction intensity shows a tendency toincrease. In particular, it can be seen that, by making CVD-W filmthickness less than 1 kÅ (or further, less than 0.5 kÅ), and making itssurface roughness (Ra) less than 100 Å (or further, less than 65 Å), Al(111) diffraction intensity will increase.

Crystal direction in the CVD-W/AlSiCu stacked film assembly, when CVD-Wfilm thickness was fixed (1 kÅ) and surface roughness was changed, wasexamined using the XRD (Θ-2Θ) method; the results are shown in FIG. 4.In addition, from these results, the relationship between CVD-W filmsurface roughness and Al (111) diffraction intensity is shown in FIG. 2.According to this, Al (111) diffraction intensity shows a tendency toincrease as CVD-W film surface roughness decreases. In particular, itcan be seen that, by using the I.C. method or (NUC.+V.F.) method as themethod of film formation, the surface roughness Ra of CVD-W film can beset at less than 100 Å (or further, less than 65 Å), and Al (111)diffraction intensity will be increased.

Here, since it is difficult to explain crystal orientationcharacteristics only with the Θ-2Θ method, these orientationcharacteristics will be accurately explained using a pole graphic method(Schulz reflection method) as shown in FIG. 8. Pole measurement is amethod in which diffraction intensity for a specific crystal face foreach orientation in a sample is determined, and this crystal face poledistribution density is represented by a diffraction intensityequivalent height line using stereo photography. With positive polemeasurement, the Schulz reflection method is used. Θ-2Θ is fixed to theBragg angle of the desired crystal face, and by scanning at angle daround the sample normal line and at angle a around the horizontal axis,crystal face diffraction intensity for the desired direction ismeasured.

From the data in FIG. 8, and from the data in FIG. 9, viewed at T.D.(transverse direction) orientation cross section, it can be seen thatthe Al was oriented to (111). From the maximum intensity difference forthese orientation characteristics observed at α=90°, it was verifiedthat the Al (111) orientation increases as CVD-W film surface roughnessdecreases.

On the other hand, the presence or absence of lattice matching of theCVD-W film and AlSiCu film will be explained. In samples where CVD-Wfilm thickness was changed, an increase in Al (111) orientation was seenwith a corresponding reduction in CVD-W (200) diffraction intensity. InCVD-W films where film thickness was fixed (1 kÅ) and film formationconditions were changed, this correlation was not observed. The Alcrystal is an FCC (face centered cubic) structure and its latticeconstant is a =4.0497 Å. The CVD-W crystal is a BCC (body centeredcubic) structure and its lattice constant is a=3.1653 Å. So, it is clearthat there was no matching of CVD-W (200) and Al (111).

From the above, Al (111) orientation in the CVD-W/AlSiCu stacked thinfilm assembly depends on CVD-W surface roughness, and the unevenness ofthe underlying layer affects the Al crystal direction. Thus, bycontrolling surface roughness of the underlying CVD-W film so that it isless than 100 Å, in accordance with this invention, it is possible tocontrol Al (111) orientation, and so it will be possible to form thinfilms that will produce increased reliability, e.g., EM resistance. Inaddition, Al (111) diffraction intensity showed a tendency to increaseas the CVD-W film became thinner. In addition, Al crystal grain diameterwas about 1 μm, with no significant dependence on CVD-W film thickness.

When Al alloy film is formed on CVD-W film, the effect of the underlyingCVD-W film surface conditions on crystal orientation of the Al alloyfilm is thought to be as shown representationally in FIG. 10.

First, in the nucleus formation process, with an increase in movementenergy of condensed atoms, an orientation as shown by the arrows isexhibited. As in Thornton's report, orientation will occur so that facesparallel to the surface will become the most densely packed faces. Thus,in Al, (111) orientation is seen. In CVD-W, when surface roughness islarge, as in FIG. 10, there will be variation in the crystal directionof the nuclei formed. In contrast to this, with a flat surface, therewill be no variation in the crystal direction of the nuclei formed.

Accompanying nucleus growth, connected crystal grains will be provided.The disappearance of grain boundaries will be caused by crystal graingrowth, and one crystal grain direction will be exhibited. Crystal graingrowth and grain boundary disappearance are repeated. On the CVD-W film,crystal grain growth and grain boundary disappearance are repeated whilevariation in crystal direction during nucleus formation is maintained.In contrast to this, since there is no variation in crystal directionduring nucleus formation with a flat surface, crystal grain growth andgrain boundary disappearance are repeated while this is maintained.

Even with crystal growth by annealing, initial crystal directionvariation during nucleus formation will not be eliminated.

From this analysis, making the surface of the underlying CVD-W film asflat as possible is ideal, and in the range where Ra≦100 Å, according tothis invention, sufficient Al (111) orientation can be realized.

Next, results of observing the grain conditions in the aforementionedCVD-W/Al stacked film assembly will be explained with reference to FIGS.11-23.

Concerning lattice matching of the CVD-W and Al, the CVD-W film has both(110) and (200), from the results of the aforementioned XRD (Θ-2Θ)method). Orienting ability is poor, and it can be considered to nearlybe a randomly oriented polycrystalline grain. When Al is deposited onthis type of CVD-W film, it is difficult to consider the crystal growthas matching the direction of the underlying CVD-W film. In fact, whenthe crystal structure and lattice constant of W and the crystalstructure and lattice constant of Al are compared, it is obvious that Al(111) does not match W (110) and (200).

Looking at the surface TEM photographs of TiW/CVD-W/AlSiCu (after theaforementioned heat treatment) shown in FIGS. 11-14, the CVD-W crystalgrains and Al crystal grains can be viewed simultaneously. Compared tothe CVD-W film crystal grains, the Al crystal grains are about 5-10times larger. It is also suggested by this that there is no latticematching.

Next, the determination of the Al surface topology is discussed. Asshown in FIGS. 15 and 16, it is clear that Al crystal grain diameterobtained from TEM photographs and Al surface unevenness, observed usingAFM, judged to be crystal grains, do not agree. It can be recognizedthat Al surface topology is not that of the Al crystal grain unitperiod, but rather has a period near the CVD-W crystal grain diameter.Thus, it is believed that Al surface topology follows the unevennesscaused by CVD-W surface crystal grains.

Distinct boundaries (grain boundaries) are observed in each of the TEMphotographs shown in FIGS. 11-14. In contrast to this, the discerning ofboundaries in the AFM images (FIG. 17) is somewhat difficult, and in theresults of measuring grain size, a difference in the average value inthe degree of magnification is recognized.

Next, what determines Al crystal grain diameter and crystal orientationis discussed. From FIGS. 15-17, in TiW (0.6 kÅ)/CVD-W (1.5-3.0kÅ)/AlSiCu (6.0 kÅ), as the CVD-W film increases from 1.5 kÅ to 3.0 kÅin thickness, Al crystal grain diameter shows a tendency to be reducedfrom 0.94 μm to 0.65 μm. In contrast to this, from FIGS. 5 and 6, in TiW(0.6 kÅ)/CVD-W (0.5-2.0 kÅ)/AlSiCu (5.0 kÅ), almost no change wasobserved in Al crystal grain diameter, which remained at 1.0 μm, evenwhen the thickness of the CVD-W film increased from 0.5 kÅ to 2.0 kÅ. Asshown in FIG. 1, however, Al (111) diffraction intensity showed atendency to fall.

From the AFM data shown in FIGS. 18-23, it can be seen that, as CVD-Wfilm thickness increases, surface roughness increases. In addition, itcan be seen that surface roughness can be changed by changing CVD-W filmformation conditions while keeping film thickness constant.

From the data in each of the tests shown above, it is thought that allthe CVD-W film characteristics generally change in the directions shownin Table I below, depending on film thickness.

                  TABLE I                                                         ______________________________________                                        CVD-W FILM  THICKNESS                                                                      ##STR1##                                                         ______________________________________                                        CVD-W SURFACE  ROUGHNESS                                                                   ##STR2##      *increases with film   thickness                   Al CRYSTAL GRAIN  DIAMETER                                                                 ##STR3##      *decreases when film   thickness is greater                                   than 2 kÅ                                      Al (111) CRYSTAL  ORIENTATION                                                              ##STR4##      *decreases with film   thickness, increases                                   when film thickness   is less than 1               ______________________________________                                                                   kÅ                                         

In short, with surface roughness (Ra=less than 200 Å) that correspondsto CVD-W film thickness of less than 2.0 kÅ, no change in Al crystalgrain diameter is seen and an increase in Al (111) orientation is seen.This tendency is especially noticeable when CVD-W film thickness is lessthan 1.0 kÅ (surface roughness Ra=100 Å or less) (refer to FIG. 1).Conversely, Al crystal grain diameter will become smaller and Al (111)orientation will also deteriorate for surface roughness that correspondsto CVD-W film thickness that exceeds 2.0 kÅ.

An X-ray diffraction spectrum like that shown in FIG. 24 was obtainedpertaining to how crystal orientation characteristics change relative tochanges in AlSiCu film thickness (AlSiCu film thickness interdependencewith TiW/CVD-W/AlSiCu film crystal orientation).

Based on this, it is seen that the Al (111) peak will be larger with anincrease in AlSiCu film thickness. It is believed that this is due tothe fact that, as film thickness increases, crystal growth progresses,crystal grain diameter grows larger, and crystal orientationcharacteristics improve.

Next, evaluation of the actual characteristics of stacked filmassemblies constructed in accordance with this invention will beexplained.

(1) EM resistance:

EM resistance, when CVD-W surface roughness was decreased and Al (111)crystal orientation increased by changing film thickness from 2.0 kÅ to0.5 kÅ, was measured under the following conditions. The resultsobtained are the data shown in Table 2 below: MTF (generally expresseslife until wiring disconnects=average breakdown time, but [here]expressed by the time needed for changes in wiring resistance to reach2%, 6% and 11%.) Here, in addition, ARC-TiW (film to prevent halationduring photoetching) was provided on the AlSiCu.

Sample: TiW (0.6 kÅ)/CVD-W (0.5-2.0 kÅ)/AlSiCu (5.0 kÅ)/ARC-TiW (0.2 kÅ)

Test conditions: current density=3×106 Å/cm2

temperature=150° C.

stacked film line width=2.0 μm

stacked film length=10 μm

                  TABLE II                                                        ______________________________________                                        MTF [hours] (standard deviation)                                                                      Rate of                                                            Rate of    increase in                                                                             Rate of                                                  increase in                                                                              resistance:                                                                             increase in                                              resistance: 2%                                                                           6%        resistance: 11%                             ______________________________________                                        CVD-W:0.5KÅ(Ra 63Å)                                                                77.9(0.26) 121.1(0.28)                                                                             159.0(0.29)                                 CVD-1:1.0KÅ(Ra 95Å)                                                                43.5(0.19) 63.8(0.17)                                                                              80.3(0.16)                                  CVD-W:2.0KÅ(Ra197Å)                                                                24.3(0.17) 37.2(0.16)                                                                              47.8(0.14)                                  ______________________________________                                    

As is clear from these results, if CVD-W film surface roughness (Ra) iskept to less than 100 Å, or further, to less than 65 Å, by controllingfilm thickness in accordance with this invention, MTF is greatlyincreased, and it can be seen that EM resistance is remarkably improved.Furthermore, MTF standard deviation hardly changes with each of theseconditions. These results can be considered to agree qualitatively withthe previously mentioned equation of S. Vaidya. In short, it isconcluded that EM resistance can be improved by decreasing CVD-W surfaceroughness and increasing Al (111) crystal orientation. (2) VIAresistance (contact resistance):

0.5 μm contact holes were provided and VIA contact (underlying wiringwas a CVD-W film) in stacked film assemblies based on this invention wasmeasured. As shown in FIG. 25 (in the figure, G-CLEAN indicates itemscleaned with very low concentration hydrofluoric acid), even when CVD-Wfilm thickness was changed in the range of from 2.0 kÅ to 0.5 kÅ, VIAcontact rose no more than between 1.2 Ω and 2.1 Ω, with no greatdifference in this change. It is judged that with CVD-W Ra≦100 Å (withfilm thickness ≦1.0 kÅ), resistance will also be in a usable range.

(3) Step coverage (rate of step coverage):

The results shown in Table III below were obtained when stacked filmassemblies based on this invention were formed for 0.5 μm contact holes.

                  TABLE III                                                       ______________________________________                                               CVD-W:500 Å                                                                        CVD-W:1000 Å                                                                           CVD-W:2000 Å                                        THICKNESS                                                                              THICKNESS    THICKNESS                                        ______________________________________                                        Hole side wall                                                                         66.7%      90.9%        90.9%                                        Hole bottom                                                                            100%       100%         100%                                         section                                                                       ______________________________________                                    

From these results, it can be seen that there is a tendency for the stepcoverage rate to fall when CVD-W film thickness is reduced from 2 kÅ to0.5 kÅ, but even when CVD-W Ra≦100 Å (with film thickness ≦1.0 kÅ) basedon this invention, sufficient step coverage, from an applicationstandpoint, is exhibited.

The stacked film assembly based on this invention as described above canbe installed as wiring in a semiconductor device, as shown in FIG. 26.Here, with halation prevention film 34, indicated by the imaginarylines, formed of TiW, etc., the reflection of exposure light duringpatterning of the wire by photoetching can be reduced (with TiW, areflection rate of no more than 45% to Al 100%, and with TiN, areflection rate of no more than 15%). The film can be exposed in thetarget pattern with a photoresist, and wire patterning precision will beincreased as a result. Although this halation prevention film 34 ispreferable, it is omitted from the figures below.

FIG. 27 shows a DRAM (64MB application, for example) memory cell inwhich a stacked film assembly based on this invention has beeninstalled. In FIG. 27, the memory cell comprises an n⁺ source region 40and an n⁺ drain region 22, a gate oxide film 41, a polysilicon gateelectrode 42, an SiO₂ film 43, a nitride film 44 produced by a side walltechnique, an insulation layer 45, an interlayer insulation film 46, astorage node 47, a dielectric film 48, a field plate 49, and aninterlayer insulation film 50.

This DRAM is made with a capacitor C connected to source region 40, in astacked construction. This capacitor may be the well-known RPSTT(reverse plate stacked trench) construction, or constructed by stackingstorage node polysilicon-dielectric film-field plate sequentially insidethe trench by a normal STT (stacked in trench) procedure.

Then, the stacked film assembly based on this invention is adhered tocontact hole 4 on drain region 22 as the aforementioned blanket type ofwiring already described and is installed as the bit line on interlayerinsulation film (SIO₂) 50; it is extremely effective for a highlyintegrated device, because of the outstanding characteristics alreadymentioned.

FIG. 28 shows wiring made of the stacked film assembly based on thisinvention installed on insulation layer 51 as blanket type film. Thisfigure is simplified, and there may be various films or wires beneathinsulation layer 51. In addition, wiring made from the aforementionedstacked film assembly can be used as a multilayer wiring and it may beformed so that top and bottom wires are connected as upper or lowerlayer wiring via through-holes (omitted from the figure). When theaforementioned stacked film assembly is applied as an upper layer wiringon the through-hole, it can be formed in the same way as by adhesion tothe contact hole shown in FIG. 26.

When the stacked film bottom layer CVD-W based on this invention will bethe blanket type, the NUC. method or (NUC.+V.F.) method described aboveis appropriate. This allows uniform W growth in the bottom section andside wall sections of the contact hole by the seeding described aboveusing SiH₄ feed. In this case, adhesive film 31 should be provided togive sufficient adhesion.

FIG. 29 is an example of selective formation that is the opposite of theblanket type described above. When a CVD-W film is formed at a lowtemperature (200° C.-300° C.) at low pressure (100-1 mtorr), theelectronegativity of WF₆ is high, it is adsorbed onto the silicon, andelectron replacement occurs readily. For this reason, CVD-W film 33 canbe selectively adhered directly onto silicon (in concrete terms,semiconductor region 22 of contact hole 4).

Although specific embodiments of this invention have been described,further variations of the described embodiments are possible on thebasis of the technical concepts of this invention.

For example, the film formation conditions may be changed in variousways, including the stacked materials and number of layers in thestacked film assembly described above. In addition to CVD-W, CVD-TiN,CVD-Ti, etc. can be used for film formation.

In addition, the stacked film assembly based on this invention, asidefrom the goals mentioned above, also has a wide range of otherapplications when a stacked thin film construction is used, for example,when crystal orientation control of the top thin film is necessary forthe purpose of obtaining certain characteristics, or when crystalorientation of the top thin film must be controlled by controlling thesurface roughness of the underlying layer. It is also applicable to avariety of devices.

In accordance with the invention, a stacked film assembly to serve aswiring in a semiconductor device is constructed so that the surfaceroughness (Ra) of the underlying film will be less than 100 Å and sothat crystal orientation of the top film formed on this surface will becontrolled. Thus, even with no lattice matching of the bottom and topfilms, crystal orientation of the top film can be sufficientlycontrolled to reach a target ((111) face with aluminum film), and inparticular it will be possible to readily construct stacked filmassemblies having a satisfactory barrier function as well as sufficientEM resistance and with good film formation.

We claim:
 1. A method for making a stacked film assembly including atleast a top film and a bottom film, said method comprising:providing afirst film in an area, the first film being intended to serve as the topfilm of the stacked film assembly; feeding a gaseous chemical compoundin which the chemical elements required for the formation of a secondfilm are contained into the area in which the first film is disposed,the second film being intended to serve as the bottom film of thestacked film assembly; breaking down the gaseous chemical compound inwhich the chemical elements for the formation of the second film arecontained; and forming the second film onto the first film by chemicalvapor deposition of the chemical elements to be included in the secondfilm as provided by the breaking down of the gaseous chemical compound,while controlling the surface roughness (Ra) of the second film incontact with the first film to be no greater than 100 Å.
 2. A method ofmaking a stacked film assembly as set forth in claim 1, wherein thebreaking down of the gaseous chemical compound containing the chemicalelements from which the second film of the stacked film assembly is tobe made is accomplished by incorporating the gaseous chemical compoundin a circulation gas having feed-limiting conditions causing thebreakdown of the gaseous chemical compound to provide the chemicalelements from which the second film of the stacked film assembly is tobe made.
 3. A method of making a stacked film assembly as set forth inclaim 1, wherein the breaking down of the gaseous chemical compoundcontaining the chemical elements from which the second film of thestacked film assembly is to be made is accomplished by incorporating thegaseous chemical compound in a highly reactive first circulation gas;anddecomposing the resultant gaseous chemical compound after exposure tothe first circulation gas under surface reaction limiting conditions byincorporating the resultant gaseous chemical compound after exposure tothe first circulation gas in a second circulation gas that is lessreactive as compared to the first circulation gas to provide availablechemical elements for the formation of the second film.
 4. A method ofmaking a stacked film assembly as set forth in claim 2, furtherincluding forming the first film which is to serve as the top film inthe stacked film assembly by one of the techniques including sputtering,chemical vapor deposition, and vapor deposition.
 5. A method of making astacked film assembly as set forth in claim 3, further including formingthe first film which is to serve as the top film in the stacked filmassembly by one of the techniques including sputtering, chemical vapordeposition, and vapor deposition.
 6. A method of making a stacked filmassembly as set forth in claim 3, wherein the formation of the secondfilm of the stacked film assembly is accomplished by providing a firstchemical, vapor deposition procedure to produce a first film portion ofthe second film from the chemical elements of the gaseous chemicalcompound when employed with the highly reactive first circulation gas;andproviding a second chemical vapor deposition procedure to produce asecond film portion of the second film from the chemical elements of thegaseous chemical compound when employed with the less reactive secondcirculation gas.
 7. A method of making a stacked film assembly as setforth in claim 6, wherein the first and second chemical vapor depositionprocedures provide relative thicknesses of the first and second filmportions of the second film in the range of (5:5)-(3:7).